Free radicals formed by H(Mu) addition to fluoranthene Jean-Claude Brodovitch, Brenda Addison-Jones, Khashayar Ghandi, Iain McKenzie, Paul W. Percival, and Joachim Schüth Abstract: Muonium has been used as an H atom analogue to investigate the free radicals formed by H addition to the polyaromatic hydrocarbon fluoranthene. There are nine unique carbons in the molecule, but only five radicals were de- tected. Muon and proton hyperfine constants were determined by transverse field μSR and μLCR, respectively, and compared with calculated values. All signals were assigned to radicals formed by Mu addition to C-H sites. There is no evidence for addition to the tertiary carbons at ring junctions. Key words: muonium, fluoranthene, free radical, hyperfine constants. Résumé : Les radicaux libres formés par addition de l’atome d’hydrogène à la molécule aromatique du fluoranthène ont été étudiés en utilisant le muonium comme analogue de l’atome d’hydrogène. Il y a neuf carbones non-équivalents présents dans la molécule, toutefois, seulement cinq radicaux libres ont été détectés. Les constantes hyperfines du muon et de certains des protons ont pu être déterminées par spectroscopies μSR et μLCR et les valeurs obtenues ont été comparées à des valeurs calculées. Tous les signaux observés ont pu être attribués aux radicaux libres formés par l’addition de Mu aux carbones portant le groupe C-H; l’addition de Mu aux carbones tertiaires qui joignent les cycles aromatiques ne se fait pas. Mots clés : muonium, fluoranthène, radicaux libres, constantes hyperfines. Brodovitch et al. 6 Introduction One of the interesting questions raised in the past decade of research on fullerenes is how reactivity to radical attack is influenced by curvature of a carbon skeleton (1–3). Consider the addition of a single H atom or other small radical to C 60 and to C 70 . In the former case there is only one possible rad- ical product, since all carbons are identical. In contrast, the ellipsoidal C 70 has five chemically inequivalent sets of car- bon atoms, and therefore five distinct radicals could be formed. Of these, the least reactive is the carbon at the “equator” where the curvature is least. Since high reactivity is associated with release of strain energy at a site of high curvature (1), it follows that planar carbon sites would be least reactive. To test the reactivity of “flat” polyaromatic hydrocarbons (PAHs) we investigated Mu or H atom addition to pyrene, whose carbon skeleton can be considered a fragment of C 70 (4). Three radicals were identified, but all are formed by Mu addition to secondary carbons at the edge of the molecule, in the same way that cyclohexadienyl is formed by H or Mu addition to benzene. No evidence was found for addition at the tertiary carbon sites that most resemble those of a fullerene. The present study is an extension of our investigation to fluoranthene (Fig. 1). Although the PAH itself is planar, its carbon skeleton is a common fragment of fullerenes where isolated five-membered rings are surrounded by 6-rings. Since this feature introduces curvature to a fullerene, its presence might promote radical addition at ring junctions in PAHs. As far as we know there is no literature data on mono- hydrofluoranthene radicals. Muon spin rotation and muon level-crossing resonance experiments Experiments were performed at the M20 muon beam line of the TRIUMF cyclotron facility in Vancouver, BC. The sample was pure fluoranthene sealed oxygen-free in a stain- less steel vessel fitted with a thin steel foil window. Its tem- perature was maintained at ≈ 117°C (above the melting point of fluoranthene) by passing fluid from a constant tempera- ture bath (set at 120°C) through the sample mount, which was surrounded by vacuum. A liquid sample was needed to ensure sharp spectral features by averaging anisotropic hyperfine parameters. The alternative to a neat liquid is to use a solution, as in the previous study (4), but this generally leads to less intense radical signals, as a fraction of the inci- dent muons end up in the solvent. Transverse field muon spin rotation ( μSR) and muon (avoided) level-crossing resonance (LCR) spectra were accu- mulated over a period of several days. Apart from the higher Can. J. Chem. 81: 1–6 (2003) doi: 10.1139/V02-191 © 2003 NRC Canada 1 Received 18 October 2002. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 14 January 2003. J.-C. Brodovitch, B. Addison-Jones, K. Ghandi, I. McKenzie, P.W. Percival, 1 and J. Schüth. 2 Department of Chemistry and TRIUMF, Simon Fraser University, Burnaby, BC V5A 1S6, Canada. 1 Corresponding author (e-mail: percival@sfu.ca). 2 Present address: debis Systemhaus Information Security Services GmbH, Rabinstrasse 8, 53111 Bonn, Germany.